'Diamond' in the Bible

The sin of Judah is engraved with an iron chisel on their stone-hard hearts. It is inscribed with a diamond point on the horns of their altars.

I have made your forehead harder than flint -- like diamond! Do not fear them or be terrified of the looks they give you, for they are a rebellious house."

Indeed, they made their heart as hard as diamond, so that they could not obey the Torah and the other words the Lord who rules over all had sent by his Spirit through the former prophets. Therefore, the Lord who rules over all had poured out great wrath.